Sometimes, if an Ubuntu installation goes wrong, or a Windows bootloader overwrites MBR and doesn't recognise the Linux installation, we have to restore our GRUB bootloader

we boot the Live CD, we choose the first option and in a few minutes we have arrived at the Live CD Desktop.

So, we go : Applications --> Accessories --> Terminal

Then, we have to remember which is our Ubuntu installation partition.

In our example, it is the second one (/dev/sda2), formatted as ext3, in the first HDD of a SATA controller. We suppose that it is the second one, since, in case we have Windows that demand to be in the first partition (/dev/sda1), this one is occupied.

Now, you have to be really careful. You have to enter the right partition, instead of sda2 (unless it is the same) In the terminal :

cd /

sudo -s -H

mount -t ext3 /dev/sda2 /mnt

mount -t proc proc /mnt/proc

mount -t sysfs sys /mnt/sys

mount -o bind /dev /mnt/dev

chroot /mnt /bin/bash

And now, you are actually "running" Ubuntu within the Hard Drive but through Live CD's terminal.

Now we restore GRUB like that:

1) Restoration to MBR

grub-install /dev/sda

2) Restoration to partition (example: /dev/sda2)

grub-install /dev/sda2

In the first case (that is the most usual) you have certainly installed GRUB on MBR after you receive, in the terminal, the message that there are no errors.